The present invention relates to a positive electrode active material for a non-aqueous electrolyte battery. The invention also relates to a high-capacity and low-cost non-aqueous electrolyte secondary battery having a positive electrode containing a specific positive electrode active material.
In recent years, with the widespread use of cordless and portable AV appliances and personal computers, the need has been increasing for compact, light weight, and high energy density battery batteries as their power sources. In particular, lithium secondary batteries, because of their high energy density, are expected to be a dominant battery in the next generation battery, and their potential market is very large.
In most of the lithium secondary batteries currently available on the market, LiCoO2 having a high voltage of 4 V is used as the positive electrode active material, but LiCoO2 is costly because Co is expensive. Under these circumstances, research has been progressing to investigate various positive electrode active materials as substitutes for LiCoO2. Among them, lithium-containing transition metal oxides has been wholeheartedly researched, and LiNiaCobO2 (a+b≈1) is promising, and at present, it seems that LiMn2O4 having a spinel structure has already been commercialized.
In addition, nickel and manganese as substitutes for expensive cobalt are also being researched vigorously.
LiNiO2 having a layered structure, for example, is expected to have a large discharge capacity, but its crystal structure changes during charging and discharging and, therefore, it deteriorates rapidly. In view of this, it is proposed to add to LiNiO2 an element that can stabilize the crystal structure during charging and discharging and thus prevent the deterioration. As the element to be added, there are exemplified, cobalt, manganese, titanium and aluminum, for example.
Prior art examples that use composite oxides of Ni and Mn as the positive electrode active material for lithium secondary batteries will be described below.
U.S. Pat. No. 5,393,622, for example, proposes a method in which a hydroxide of Ni, a hydroxide of Mn and a hydroxide of Li are dry-mixed together and baked and, after cooling them down to room temperature, the mixture is again heated and baked to obtain an active material having a composition represented by the formula LiyNi1xe2x88x92xMnxO2 wherein 0xe2x89xa6xxe2x89xa60.3, 0xe2x89xa6yxe2x89xa61.3.
Further, U.S. Pat. No. 5,370,948 proposes a method in which a Li salt, a Ni salt and a Mn salt are mixed together into an aqueous solution, followed by drying and baking, to obtain an active material represented by the formula LiNi1xe2x88x92xMnxO2 wherein 0.005xe2x89xa6xxe2x89xa60.45.
Further, U.S. Pat. No. 5,264,201 proposes a dry synthesis method in which hydroxides or oxides of nickel and manganese and an excess amount of lithium hydroxide are mixed together and baked, and a synthesis method in which an oxide of nickel and manganese is added to a saturated aqueous solution of lithium hydroxide to form a slurry and the slurry is then dried and baked under a reduced pressure, to obtain an active material represented by the formula LixNi2xe2x88x92xxe2x88x92yMnyO2 wherein 0.8xe2x89xa6xxe2x89xa61.0, yxe2x89xa60.2.
Furthermore, U.S. Pat. No. 5,629,110 proposes a dry mixing synthesis method which uses xcex2-Ni(OH)2 to obtain an active material represented by the formula LiNi1xe2x88x92xMnxO2 wherein 0 less than xxe2x89xa60.2, yxe2x89xa60.2.
Japanese Unexamined Patent Publication No. Hei 8-171910 proposes a method in which manganese and nickel are coprecipitated by adding an alkaline solution into an aqueous solution mixture of manganese and nickel, then lithium hydroxide is added and the resulting mixture is baked, to obtain an active material represented by the formula LiNixMnxxe2x88x921O2 wherein 0.7xe2x89xa6xxe2x89xa60.95.
Further, Japanese Unexamined Patent Publication No. Hei 9-129230 discloses a preferred particulate active material having the composition represented by the formula LiNixMxxe2x88x921O2 wherein M is at least one of Co, Mn, Cr, Fe, V and Al, 1 greater than xxe2x89xa60.5, and shows a material with x=0.15 as the active material containing Ni and Mn.
Further, Japanese Unexamined Patent Publication No. Hei 10-69910 proposes an active material synthesized by a coprecipitation synthesis method, represented by the formula Liyxe2x88x92x1Ni1xe2x88x92x2MxO2 wherein M is Co, Al, Mg, Fe, Mg or Mn, 0 less than x2xe2x89xa60.5, 0xe2x89xa6x1 less than 0.2, x=x1+x2, and 0.9xe2x89xa6yxe2x89xa61.3. This patent publication describes that the discharge capacity is inherently small if M is Mn, and the original function of the positive electrode active material intended to achieve a high-capacity lithium secondary battery is dismissed if x2 is more than 0.5. LiNi0.6Mn0.4O2 is exemplified as a material having the highest proportion of Mn.
U.S. Pat. No. 5,985,237 discloses a production method for LiMnO2 having a layered structure, but this is essentially a 3 V level active material.
All the prior art examples disclosed in the above U.S. Patents and Japanese Unexamined Patent Publications are intended to improve the electrochemical properties such as the cycle characteristics of LiNiO2 by adding a trace amount of an element into LiNiO2, while retaining the characteristic properties of LiNiO2 itself. Accordingly, in the active material obtained after the addition, the amount of Ni is always larger than that of Mn, and the proportion of Ni:Mn=0.8:0.2 is proposed in many cases. As an example of a material having a proportion with a highest amount of Mn, Ni:Mn=0.55:0.45 is disclosed.
However, in any of these prior art examples, it is difficult to obtain a composite oxide having a single-phase crystal structure since LiNiO2 is separated from LiMnO2. This is because Mn2+ tends to be oxidized to Mn3+ during coprecipitation and Mn3+ is difficult to form a homogenous composite oxide with Ni2+.
As described above, as a substitute material for the currently commercialized high voltage 4V LiCoO2.LiNiO2 and LiMnO2 as high-capacity and low-cost positive electrode active materials having a layered structure like LiCoO2 has been researched and developed.
However, the discharge curve of LiNiO2 is not flat, and the cycle life is short. In addition, the heat resistance is low, and using this material as a substitute material for LiCoO2 would involve a severe problem. In view of this, improvements have been attempted by adding various elements to LiNiO2, but satisfactory results have not been obtained yet. Further, it is only possible to obtain a voltage of 3 V with LiMnO2 therefore, low-capacity LiMn2O4 which does not have a layered structure but has a spinel structure is beginning to be researched.
Accordingly, an object of the present invention is to provide a positive electrode active material capable of solving the above-mentioned problems. Also, another object of the invention is to obtain a positive electrode active material that has a voltage of 4 V equivalent to that of LiCoO2, exhibits a flat discharge curve, and is higher in capacity and lower in cost than LiCoO2. Further, still another object of the present invention is to provide a non-aqueous electrolyte secondary battery using such a positive electrode active material and achieving a high capacity and excellent charge/discharge efficiency.
The present invention relates to a positive electrode active material for a non-aqueous electrolyte battery comprising a crystalline particle of an oxide, said oxide containing nickel element and manganese element in substantially the same atomic ratios and having a crystal structure of a rhombohedral structure, which belongs to rhombohedral crystal system.
In other words, in the crystalline particle, nickel atoms and manganese atoms are uniformly or homogeneously dispersed.
It is effective that the crystal structure of the crystalline particle belongs to the hexagonal crystal system, and the length of c-axis is not shorter than 14.25 angstroms.
It is effective that the oxide further contains a lithium element.
It is effective that the crystalline particle is in the shape of sphere.
Further, it is effective that the positive electrode active material comprises a mixture of a crystalline particle of the oxide having a particle size of 0.1 to 2 xcexcm and a secondary particle of the crystalline particle having a particle size of 2 to 20 xcexcm.
Further, it is preferable that the crystal structure of the crystalline particle is a rhombohedral structure in which the volume of a unit lattice decreases through oxidation.
Further, it is effective that an error range between an atomic ratio of nickel element and an atomic ratio of manganese elements is within not larger than 10 atomic percent.
Further, it is effective that the lithium element, nickel element and manganese element contained in the oxide satisfy the relation: 0.97xe2x89xa6Li/(Ni+Mn)xe2x89xa61.03.
Further, the present invention also relates to a method for producing a positive electrode active material for a non-aqueous electrolyte battery, comprising the steps of:
introducing an aqueous solution containing a nickel salt and a manganese salt and an alkaline solution simultaneously into a reactor, and coprecipitating the nickel and the manganese while passing an inert gas therethrough to obtain a nickel manganese hydroxide and/or a nickel manganese oxide;
mixing the nickel manganese hydroxide and/or the nickel manganese oxide with a lithium compound to obtain a mixture; and
baking(sintering) the mixture to obtain a positive electrode material.
In the production method, it is effective that the temperature of the reactor is 30 to 50xc2x0 C.
Further, it is effective that each of the nickel salt and the manganese salt is sulfate.
Further, it is effective that the alkaline solution is an aqueous solution containing a mixture of sodium hydroxide and ammonia water.
Further, it is effective that the lithium compound is lithium carbonate and/or lithium hydroxide.
Further, it is effective that the temperature of the baking is not lower than 550xc2x0 C.
Further, it is effective that the temperature of the baking is not lower than 950xc2x0 C., and the mixture after the baking is subsequently baked again at a temperature of 700 to 780xc2x0 C.
The present invention further relates to a method for producing a positive electrode active material for a non-aqueous electrolyte battery, comprising the steps of:
dry mixing LiOH.H2O, Ni(OH)2 and MnOOH, each having a particle size of not larger than 0.3 xcexcm to obtain a mixture; and
baking the mixture to obtain a positive electrode active material.
In this invention, the temperature of the baking is not lower than 550xc2x0 C.
The present invention also relates to a non-aqueous electrolyte secondary battery comprising: a negative electrode containing, as a negative electrode active material, metallic lithium and/or a substance at least capable of absorbing (intercalate) and desorbing (deintercalate) lithium ion; a separator; a positive electrode containing the above-mentioned positive electrode active material; and an electrolyte.
In accordance with the present invention, a non-aqueous electrolyte battery can be provided that can effectively utilize an inexpensive nickel manganese composite oxide as the positive electrode active material, and that achieves a high capacity and excellent charge/discharge efficiency.
While the novel features of the invention are set forth particularly in the appended claims, the invention, both as to organization and content, will be better understood and appreciated, along with other objects and features thereof, from the following detailed description taken in conjunction with the drawings.